We are continuing to probe the molecular basis of immune recognition through the application of modern techniques of experimental cellular immunology to the genetically defined murine model. The extremely well-characterized protein antigen, hen egg-white lysozyme (HEL), other avian and mammalian lysozymes, peptide fragments of HEL, specifically derivatized HEL, and synthetic peptides related to HEL are all utilized in this study. At each level of recognition there appears to be severe immune focusing to a highly restricted region of HEL that, however, is different for each cell type: B cells, helper T cells, and suppressor T cells. In addition, the response is dominated by a single public idiotype, IdXL. At the B-cell level, we are more extensively exploring the early transient recognition by B cells of an epitope including the three N-terminal amino acids of HEL that normally appear obligatory for subsequent recognition of other epitopes by B cells. In particular, we are attempting to disrupt this ordered read-out by a number of alternative modes of immunization and pretreatments. We have shown that the ultimate epitopes recognized by B cells seem confined to a single face of HEL, and we have more carefully defined this restriction through an extensive study of the degree to which monoclonal antibodies directed against HEL show competitive binding. We have established a library of HEL-specific T-cell clones and through the use of synthetic peptides are extending our studies of the fine specificity of these clones. In particular, we are investigating changes in specificity as a function of immunizing antigen and of haplotype. We are investigating in detail the nature of IdXL-recognizing T cells raised in response to HEL. The fine specificity of T suppressor cells active in tolerance is being explored. The nature of IdXL in molecular terms has been studied through recombination studies with monoclonal antibodies, and it has been established that IdXL is dependent on light chains, whereas fine specificity is determined by heavy chains. DNA studies suggest that multiple variable region genes code for both heavy and light chains of IdXL-positive monoclonal antibodies. (LB)